Tag Archives: Higgs

On 8 October 2013, the Nobel prize for physics was awarded to Francois Englert and Peter Higgs. In one sense this was a long time coming: the theoretical work that won the prize took place in 1964 (Englert, and his late colleague Robert Brout, working independently of Higgs, published first; a few weeks later Higgs published a paper that explicitly predicted the existence of a scalar boson; another group of physicists – Gerald Guralnik, Carl Hagen and Tom Kibble – published related work later in the same year). In another sense the prize was awarded remarkably quickly: experimental proof of the existence of a fundamental boson was announced on 4 July 2012, and it wasn’t until 14 March 2013 that it was confirmed to be a scalar (spin-0) boson. (If you want to learn more about the Higgs mechanism, you can find a variety of explanations here.)

To my mind, the discovery of the Higgs is one the crowning achievements of human civilisation: it is the culmination of a process that began 2500 years ago with the Greeks. Physicists now have a standard model of fundamental particles: there exists a small number of spin-1/2 point particles (6 quarks; the electron, muon and tau each with their associated neutrino) which interact via the exchange of spin-1 particles that mediate the electroweak and strong (these exchange particles being the photon; W+, W– and Z0; 8 gluons). In the ‘pure’ theories underpinning this model the fundamental particles are massless; they acquire mass – and thus in a certain sense their very existence – by interacting with a spin-0 field that pervades the entire universe. This spin-0 field has an associated particle; the Higgs boson. And that’s it. End of story. Except…

We are really just at the beginning of the story. The theories underpinning the standard model are in conflict with the other central pillar of physics: general relativity. The standard model is based on quantum physics; general relativity is a classical theory. Physicists need to develop a quantum theory of gravity. Furthermore, we now know that the standard model applies to only 5% of the universe: 95% of the mass-energy content of the universe resides in the so-called ‘dark’ sector. We desperately need to understand the nature of dark matter and dark energy.

Now that the Large Hadron Collider has discovered the Higgs its next job, when it becomes operational again after its current upgrade, is to shed light on the dark sector.

The search for dark matter is much more difficult than the search for the Higgs. At least with the Higgs we had an idea where to look: the Higgs is part of the Standard Model of particle physics, after all, and so physicists could guess at least some of its properties. If the Higgs existed, physicists knew they could catch glimpses of it at the LHC.

It’s different with dark matter.

If dark matter particles exist then they clearly and unambiguously relate to physics beyond the Standard Model. This in turn means there are very few clues that can help physicists in the search for dark matter (a search that is incredibly difficult anyway, since dark matter interacts so rarely with “normal” matter). Since physicists by definition don’t know what it is they are looking for when they search for dark matter, that complicates the search enormously: the sort of experiment that can look for axions (one candidate dark matter particle) is very different from the sort of experiment that can look for WIMPs (another candidate).

The currently favoured explanation for dark matter is perhaps the WIMP explanation – that most of the material in the universe consists of Weakly Interacting Massive Particles. Many experiments are currently looking for WIMPs and, as explained in my book New Eyes on the Universe, the results are intriguing. Some experiments seem to have found tentative signs of a WIMP signal; others experiments have found nothing.

The XENON100 experiment uses arrays of photomultipliers, such as this one, to catch the brief flashes of light that would occur if a WIMP scattered off a xenon nucleus.(Credit: XENON100 Collaboration)

One of the biggest WIMP dark matter experiments to date as been the XENON100 collaboration, which is based deep underground at the Gran Sasso National Laboratory. The experiment employs 62kg of extremely pure liquid xenon as a WIMP target.The idea is that, once in the proverbial blue moon, a WIMP will score a direct hit on a xenon nucleus and the collision will emit small amounts of light. Sensitive detectors surrounding the liquid can detect light from the collisions.

On 18 July 2012, the collaboration announced the results from an analysis of 13 months of searching. They found no evidence that a WIMP, in all that time, had interacted with a xenon nucleus in their target.

Today’s announcement at CERN, that the CMS and ATLAS experiments have found a boson consistent with the Standard Model Higgs, is the most exciting find in particle physics since … well, since I can remember. The discovery of charm was before my time, but I was a physics student when news of the W and Z discoveries was made public and I don’t believe those announcements matched today’s press conference for drama and sheer emotion (Peter Higgs had to wipe away a tear).

This is a tremendous day for science. Just think what’s happened here. Over a period of decades, theorists and experimentalists developed a theory of the basic interactions (electromagnetism, weak force, strong force) that govern the behaviour of the fundamental particles (quarks, neutrinos, electron, muon and tau). But in order for the theory to match the observed fact that the fundamental particles have mass, theorists had to add something else into the mix. They used purely mathematical reasoning to deduce something incredible about the Universe: that it’s filled everywhere with a scalar field — the so-called Higgs field. It’s the interaction with this field that gives the fundamental particles mass.

And decades after theorists postulated the existence of this field, CMS and ATLAS have found evidence for the associated boson. They saw hints of the Higgs boson last year. Now it’s definite. It has a mass of around 125 GeV.

This is tremendous news for CMS, ATLAS, CERN and science in general. And it’s the start of a whole new era in physics. Now we know where the Higgs is, the LHC — such a tremendous machine — will be able to investigate its properties in detail. And perhaps for the first time we’ll get a glimpse beyond the Standard Model.